/**
 * Copyright 2008, David Robert Nadeau, NadeauSoftware.com
 * <p>
 * This file is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as
 * published by the Free Software Foundation, either version 2 of
 * the License, or (at your option) any later version.
 * <p>
 * This file is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 * <p>
 * You should have received a copy of the GNU Lesser General Public
 * License along with this file.  If not, see
 * <a href="http://www.gnu.org/licenses">GNU Licenses</a>.
 */
package org.hzero.metric.thread;

import java.lang.management.ManagementFactory;
import java.lang.management.MonitorInfo;
import java.lang.management.ThreadInfo;
import java.lang.management.ThreadMXBean;

/**
 * The ThreadUtilities class provides a collection of static methods
 * for listing and finding Thread, ThreadGroup, and ThreadInfo objects.
 * <p>
 * Please see the accompanying article:
 * <a href="http://nadeausoftware.com/articles/2008/04/java_tip_how_list_and_find_threads_and_thread_groups">Java tip:  How to list and find threads and thread groups</a>
 *
 * @author    <a href="http://NadeauSoftware.com/">David R. Nadeau</a>
 */
public final class ThreadUtilities {

    private static final String NULL_NAME = "Null name";
// Dummy constructor

    /**
     * The ThreadUtilities class cannot be instanced.
     */
    private ThreadUtilities() {
    }


// Thread groups
    /**
     * The root thread group saved on the first search for it.
     * The root group doesn't change for the life of the JVM,
     * so once found there is no need to find it again.
     */
    private static ThreadGroup rootThreadGroup = null;

    /**
     * Get the root thread group in the thread group tree.
     * Since there is always a root thread group, this
     * method never returns null.
     *
     * @return the root thread group
     */
    public static ThreadGroup getRootThreadGroup() {
        if (rootThreadGroup != null)
            return rootThreadGroup;

        ThreadGroup tg = Thread.currentThread().getThreadGroup();
        ThreadGroup ptg;
        while ((ptg = tg.getParent()) != null)
            tg = ptg;
        rootThreadGroup = tg;
        return tg;
    }

    /**
     * Get a list of all thread groups.  Since there is
     * always at least one thread group (the root), this
     * method never returns a null or empty array.
     *
     * @return an array of thread groups
     */
    public static ThreadGroup[] getAllThreadGroups() {
        final ThreadGroup root = getRootThreadGroup();
        int nAlloc = root.activeGroupCount();
        int n = 0;
        ThreadGroup[] groups = null;
        do {
            nAlloc *= 2;
            groups = new ThreadGroup[nAlloc];
            n = root.enumerate(groups, true);
        } while (n == nAlloc);
        ThreadGroup[] allGroups = new ThreadGroup[n + 1];
        allGroups[0] = root;
        System.arraycopy(groups, 0, allGroups, 1, n);
        return allGroups;
    }

    /**
     * Get the thread group with the given name.  A null is
     * returned if no such group is found.  If more than one
     * group has the same name, the first one found is returned.
     *
     * @param    name    the thread group name to search for
     * @return the thread group, or null if not found
     * @throws NullPointerException
     * 			if the name is null
     */
    public static ThreadGroup getThreadGroup(final String name) {
        if (name == null)
            throw new NullPointerException(NULL_NAME);
        final ThreadGroup[] groups = getAllThreadGroups();
        for (ThreadGroup group : groups)
            if (group.getName().equals(name))
                return group;
        return null;
    }


// Threads

    /**
     * Get a list of all threads.  Since there is always at
     * least one thread, this method never returns null or
     * an empty array.
     *
     * @return an array of threads
     */
    public static Thread[] getAllThreads() {
        final ThreadGroup root = getRootThreadGroup();
        final ThreadMXBean thbean =
                ManagementFactory.getThreadMXBean();
        int nAlloc = thbean.getThreadCount();
        int n = 0;
        Thread[] threads = null;
        do {
            nAlloc *= 2;
            threads = new Thread[nAlloc];
            n = root.enumerate(threads, true);
        } while (n == nAlloc);
        return java.util.Arrays.copyOf(threads, n);
    }

    /**
     * Get a list of all threads in a thread group.  An empty
     * array is returned if there are no threads in the group.
     *
     * @param    group    the thread group to list
     * @return an array of threads
     * @throws NullPointerException
     * 			if the group is null
     */
    public static Thread[] getGroupThreads(final ThreadGroup group) {
        if (group == null)
            throw new NullPointerException("Null group");
        int nAlloc = group.activeCount();
        int n = 0;
        Thread[] threads = null;
        do {
            nAlloc *= 2;
            threads = new Thread[nAlloc];
            n = group.enumerate(threads, false);
        } while (n == nAlloc);
        return java.util.Arrays.copyOf(threads, n);
    }

    /**
     * Get a list of all threads in a named thread group.
     * A null is returned if the group cannot be found.
     * An empty array is returned if there are no threads in
     * the group.
     *
     * @param    name    the name of the thread group
     * @return an array of threads, or null if the
     *			group is not found
     * @throws NullPointerException
     * 			if the name is null
     */
    public static Thread[] getGroupThreads(final String name) {
        final ThreadGroup group = getThreadGroup(name);
        if (group == null) {
            return new Thread[0];
        }
        return getGroupThreads(group);
    }

    /**
     * Get a list of all threads, sorted from highest to
     * lowest priority.  Since there is always at least one
     * thread, this method never returns null or an empty
     * array.  Since threads may change their priority during
     * this method's sort, the returned thread list may not be
     * correct by the time it is returned.
     *
     * @return an array of threads
     */
    public static Thread[] getAllThreadsPrioritized() {
        final Thread[] allThreads = getAllThreads();
        java.util.Arrays.sort(allThreads, (final Thread t1, final Thread t2) -> t2.getPriority() - t1.getPriority());
        return allThreads;
    }

    /**
     * Get a list of all daemon threads.  An empty array is
     * returned if there are no daemon threads.
     *
     * @return an array of daemon threads
     */
    public static Thread[] getAllDaemonThreads() {
        final Thread[] allThreads = getAllThreads();
        final Thread[] daemons = new Thread[allThreads.length];
        int nDaemon = 0;
        for (Thread thread : allThreads)
            if (thread.isDaemon())
                daemons[nDaemon++] = thread;
        return java.util.Arrays.copyOf(daemons, nDaemon);
    }

    /**
     * Get a list of all threads with a given thread state.
     * Thread states are defined in the Thread.State enum for
     * the Thread class.  Principal thread states include
     * RUNNABLE, WAITING, TIMED_WAITING, and BLOCKED.  An
     * empty array is returned if there are no threads in
     * the chosen state.
     *
     * @param    state    the state to look for
     * @return an array of threads in that state
     */
    public static Thread[] getAllThreads(final Thread.State state) {
        final Thread[] allThreads = getAllThreads();
        final Thread[] found = new Thread[allThreads.length];
        int nFound = 0;
        for (Thread thread : allThreads)
            if (thread.getState() == state)
                found[nFound++] = thread;
        return java.util.Arrays.copyOf(found, nFound);
    }

    /**
     * Get the thread with the given name.  A null is returned
     * if no such thread is found.  If more than one thread has
     * the same name, the first one found is returned.
     *
     * @param    name    the thread name to search for
     * @return the thread, or null if not found
     * @throws NullPointerException
     * 			if the name is null
     */
    public static Thread getThread(final String name) {
        if (name == null)
            throw new NullPointerException(NULL_NAME);
        final Thread[] threads = getAllThreads();
        for (Thread thread : threads)
            if (thread.getName().equals(name))
                return thread;
        return null;
    }

    /**
     * Get the thread with the given ID.  A null is returned
     * if no such thread is found.
     *
     * @param    id    the thread ID to search for
     * @return the thread, or null if not found
     */
    public static Thread getThread(final long id) {
        final Thread[] threads = getAllThreads();
        for (Thread thread : threads)
            if (thread.getId() == id)
                return thread;
        return null;
    }

    /**
     * Get the thread for the given thread info.  A null
     * is returned if the thread cannot be found.
     *
     * @param    info    the thread info to search for
     * @return the thread, or null if not found
     * @throws NullPointerException
     * 			if info is null
     */
    public static Thread getThread(final ThreadInfo info) {
        if (info == null)
            throw new NullPointerException("Null info");
        return getThread(info.getThreadId());
    }


// ThreadInfo

    /**
     * Get a list of all thread info objects.  Since there is
     * always at least one thread running, there is always at
     * least one thread info object.  This method never returns
     * a null or empty array.
     *
     * @return an array of thread infos
     */
    public static ThreadInfo[] getAllThreadInfos() {
        final ThreadMXBean thbean =
                ManagementFactory.getThreadMXBean();
        final long[] ids = thbean.getAllThreadIds();

        // Get thread info with lock info, when available.
        ThreadInfo[] infos;
        if (!thbean.isObjectMonitorUsageSupported() ||
                !thbean.isSynchronizerUsageSupported())
            infos = thbean.getThreadInfo(ids);
        else
            infos = thbean.getThreadInfo(ids, true, true);

        // Clean nulls from array if threads have died.
        final ThreadInfo[] notNulls = new ThreadInfo[infos.length];
        int nNotNulls = 0;
        for (ThreadInfo info : infos)
            if (info != null)
                notNulls[nNotNulls++] = info;
        if (nNotNulls == infos.length)
            return infos;    // Original had no nulls
        return java.util.Arrays.copyOf(notNulls, nNotNulls);
    }

    /**
     * Get the thread info for the thread with the given name.
     * A null is returned if no such thread info is found.
     * If more than one thread has the same name, the thread
     * info for the first one found is returned.
     *
     * @param    name    the thread name to search for
     * @return the thread info, or null if not found
     * @throws NullPointerException
     * 			if the name is null
     */
    public static ThreadInfo getThreadInfo(final String name) {
        if (name == null)
            throw new NullPointerException(NULL_NAME);

        final Thread[] threads = getAllThreads();
        for (Thread thread : threads)
            if (thread.getName().equals(name))
                return getThreadInfo(thread.getId());
        return null;
    }

    /**
     * Get the thread info for the thread with the given ID.
     * A null is returned if no such thread info is found.
     *
     * @param    id    the thread ID to search for
     * @return the thread info, or null if not found
     * @throws IllegalArgumentException
     */
    public static ThreadInfo getThreadInfo(final long id) {
        final ThreadMXBean thbean =
                ManagementFactory.getThreadMXBean();

        // Get thread info with lock info, when available.
        if (!thbean.isObjectMonitorUsageSupported() ||
                !thbean.isSynchronizerUsageSupported())
            return thbean.getThreadInfo(id);

        final ThreadInfo[] infos = thbean.getThreadInfo(
                new long[]{id}, true, true);
        if (infos.length == 0)
            return null;
        return infos[0];
    }

    /**
     * Get the thread info for the given thread.  A null is
     * returned if the thread info cannot be found.
     *
     * @param    thread    the thread to search for
     * @return the thread info, or null if not found
     * @throws NullPointerException
     * 			if thread is null
     */
    public static ThreadInfo getThreadInfo(final Thread thread) {
        if (thread == null)
            throw new NullPointerException("Null thread");
        return getThreadInfo(thread.getId());
    }


// MonitorInfo and LockInfo

    /**
     * Get the thread holding a lock on the given object.
     * A null is returned if there is no such thread.
     *
     * @param    object        the object to look for a lock on
     * @return the thread holding a lock, or
     * 				null if there is none
     * @throws NullPointerException
     * 				if the object is null
     */
    public static Thread getLockingThread(final Object object) {
        if (object == null)
            throw new NullPointerException("Null object");
        final long identity = System.identityHashCode(object);

        final Thread[] allThreads = getAllThreads();
        ThreadInfo info = null;
        MonitorInfo[] monitors = null;
        for (Thread thread : allThreads) {
            info = getThreadInfo(thread.getId());
            if (info == null)
                continue;
            monitors = info.getLockedMonitors();
            for (MonitorInfo monitor : monitors)
                if (identity == monitor.getIdentityHashCode())
                    return thread;
        }
        return null;
    }

    /**
     * Get the thread who's lock is blocking the given thread.
     * A null is returned if there is no such thread.
     *
     * @param    blockedThread    the blocked thread
     * @return the blocking thread, or null if
     * 				there is none
     * @throws NullPointerException
     * 				if the blocked thread is null
     */
    public static Thread getBlockingThread(final Thread blockedThread) {
        final ThreadInfo info = getThreadInfo(blockedThread);
        if (info == null)
            return null;
        final long id = info.getLockOwnerId();
        if (id == -1)
            return null;
        return getThread(id);
    }
}
